Kara Woo | @kara_woo Sage Bionetworks rstudio::conf 2019 BOX PLOTS A CASE STUDY IN DEBUGGING AND PERSEVERANCE 

THIS IS ONLY KIND OF ABOUT BOX PLOTS 

"When I try to produce boxplots with colours depending on a categorical variable, these appear overlapping if varwidth is set to TRUE” —GitHub user mcol 

“This should be straightforward.” 

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How do I know what the bug is? How do I know when I’m done? How do I fix it? 

HOW DO YOU KNOW WHAT THE BUG IS? Photo: Jia Ye Isolate the problem 

Photo: Sarah Dorweiler REPREX minimal reproducible example 

reprex::reprex() 

require(ggplot2) #> Loading required package: ggplot2 ggplot(data = iris, aes(Species, Sepal.Length)) + geom_boxplot(aes(colour = Sepal.Width < 3.2), varwidth = TRUE) #> Warning: position_dodge requires non-overlapping x intervals 

require(ggplot2) #> Loading required package: ggplot2 ggplot(data = iris, aes(Species, Sepal.Length)) + geom_boxplot(aes(colour = Sepal.Width < 3.2), varwidth = FALSE) 

Follow the trails Photo: Jia Ye 

require(ggplot2) #> Loading required package: ggplot2 ggplot(data = iris, aes(Species, Sepal.Length)) + geom_boxplot(aes(colour = Sepal.Width < 3.2), varwidth = TRUE) #> Warning: position_dodge requires non-overlapping x intervals 

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position_dodge() !" PositionDodge !" collide() !" pos_dodge() 

COLLIDE() Gets information about box location Looks for box overlap Passes boxes that share position to pos_dodge() POS_DODGE() Scales boxes down Places boxes side by side 

> debug(ggplot2:::collide) 

> debug(ggplot2:::collide) > ggplot(data = iris, aes(Species, Sepal.Length)) + > geom_boxplot(aes(colour = Sepal.Width < 3.2), varwidth = FALSE) #> debugging in: collide(data, params$width, #> name = “position_dodge", strategy = pos_dodge, #> n = params$n, check.width = FALSE) Browse[2]> 

> debug(ggplot2:::collide) > ggplot(data = iris, aes(Species, Sepal.Length)) + > geom_boxplot(aes(colour = Sepal.Width < 3.2), varwidth = FALSE) #> debugging in: collide(data, params$width, #> name = “position_dodge", strategy = pos_dodge, #> n = params$n, check.width = FALSE) Browse[2]> data #> ... x xmin xmax #> 1 ... 1 0.625 1.375 #> 2 ... 2 1.625 2.375 #> 3 ... 3 2.625 3.375 #> 4 ... 1 0.625 1.375 #> 5 ... 2 1.625 2.375 #> 6 ... 3 2.625 3.375 

VARWIDTH = TRUE VARWIDTH = FALSE data #> ... x xmin xmax #> 1 ... 1 0.625 1.375 #> 2 ... 2 1.625 2.375 #> 3 ... 3 2.625 3.375 #> 4 ... 1 0.625 1.375 #> 5 ... 2 1.625 2.375 #> 6 ... 3 2.625 3.375 data #> ... x xmin xmax #> 1 ... 1 0.6553988 1.344601 #> 2 ... 2 1.8750000 2.125000 #> 3 ... 3 2.7984436 3.201556 #> 4 ... 1 0.8063508 1.193649 #> 5 ... 2 1.6250000 2.375000 #> 6 ... 3 2.6599632 3.340037 

Boxes with different xmin aren’t treated as the same position. collide <- function(data, ...) { # ... plyr::ddply(data, "xmin", strategy, ..., width = width) # ... } 

HOW DO YOU FIX IT? Photo: Jia Ye Experiment 

ccc6bbb4 This doesn't fix position_dodge, but it might be in the right direction? f5946680 Commit before I break something else 

INITIAL “FIX” What’s wrong with this picture? 

•Boxes in the wrong order •Doesn’t work for continuous x axes •Incorrect scaling INITIAL “FIX” What’s wrong with this picture? 

Photo: Jia Ye Test many scenarios HOW DO YOU FIX IT? 

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Photo: Jia Ye Make it general HOW DO YOU FIX IT? 

Can we extend to bars? Can we solve other open issues? Arbitrary rectangles? 

→ POSITION_DODGE2() Used for boxes, bars, rectangles Finds overlap by comparing xmin to previous xmax → 

HOW DO YOU KNOW WHEN YOU’RE DONE? Photo: Jia Ye Again, test 

HOW DO YOU KNOW WHEN YOU’RE DONE? Photo: Jia Ye Don’t let perfect be the enemy of good 

CAN POSITION_DODGE2() REPLACE POSITION_DODGE()? 

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• Isolate the problem • Follow the trails • Experiment • Test many scenarios • Make it general • Don’t let perfect be the enemy of good https://github.com/tidyverse/ggplot2/pull/2196 

Thank you Thanks to @mcol for reporting this bug, Hadley Wickham for repeated code reviews, and Sean Kross and Karthik Ram for feedback on this presentation.